So I had an elephant man necromancer, who went to space once. They achieved this by breaking through a significant amount of of the skybox, which involved a fairly labor intensive process which combined dfhack's advfort, and dozens of adventure camps stapled next to each other, which produced a space station / space mountain highway (which stretched all the way from the world's equivalent of the grand canyon to mount Everest). Upon reaching "space" everything suddenly became very, very cold. So cold, my elephant man's eyeballs froze, as well as anything I happened to touch. I then proceeded to fall off the space station, drop all the way down into a river below, which immediately froze over and slayed my adventurer.
This was intriguing, so over the past year tried a couple times to replicate the properties of "space", or "The Vacuum", through various custom materials. And I never managed to produce anything that had the same level of effect on the environment as having a creature be exposed to vacuum of space.
With inorganics, you have access to fixed_mat_temp and spec_heat, and there's some neat stuff you can produce. To behave like a "thing with a fixed temperature", you usually set the spec_heat to 0. But if you set it at a non zero number, it'll do some weird stuff.
If you're clever, you can even exceed the cap on maximum fixed temperature Urist by using specific negative values, since the token accepts an unsigned integer and doesn't have any real sanity checks on what's going on.
You can also define a material with a specific heat of 1, and with a cold enough fixed temperature, the material will dramatically alter its temperature and actually roll over when exposed to certain environmental effects. For instance, if its picked up and then tossed into a river or body of water, the item will roll over from say, a default of 200 degrees Urist (very cold), all the way to being 59,999 degrees Urist, and produce a momentary burst of steam (note: depending on how you spawn an item made of the material, it may be important to interact with it in some way, such as picking it up first, in order for its fixed temp to become active). Upon being pulled out of the body of water, its temperature will return to being cold, and may become coated in ice.
But I've never been able to boil entire rivers or freeze them using inorganic items.
Creatures seem capable of producing constant effects though, but never to the dramatic extent that I've wanted.
I've tried tossing fire demons, lava men, and creatures made of freezing material into bodies of water, and steam will be produced constantly or whatever, but its never dramatic enough. I always defined their body materials as having fixed temperatures, and never actually considered messing around with HOMEOTHERM's effect on the environment. I always just assumed it was negligible.
Its interesting that people are reporting success here with custom creatures which are able to freeze water though, by manipulating HOMEOTHERM.